Hi BDavis. Ok, we've come well off the intended scope of my post, and we disagree on too much in your latest post for me to comment further. FWIW, I'll hold the line that Bennett's graph definitely plots refraction as a positional offset vs apparent altitude (in units of arcminutes). Refractive index is dimensionless, and is a term itself dependent on other terms (temp, pressure, humidity, etc). Main takeaway: I accept that you believe refractive index is the driving function changing focus. We just need to disagree amicably.

My original post is simple: Two readily available variables in Ekos (temperature, elevation) can be used to dramatically improve focus control. I've shown a relationship of temperature to focus. I've also shown that once factored, residuals of a temperature function can be correlated to elevation. The data speak for themselves. Anyone who wishes can verify or refute the suggested relations by characterizing their own focus behavior via the focuser debug log. I am not stating that these two functions are the perfect and complete academic explanation. Rather, these two variables are readily available in Ekos, and serve particularly well to prevent poor auto-focus starting position, offset focus position between exposures, or prevent poor focus position after slews of large elevation difference. We're trying to remain close to the focus caustic. KISS can work well here. Cheers, Doug

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